The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a method and system for optical heterodyne detection of an optical signal.
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating based OSAs and autocorrelation based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained.
As an alternative to grating based and autocorrelation based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. FIG. 1 is a depiction of a prior art optical heterodyne detection system. The optical heterodyne detection system includes an input signal 102, an input waveguide 104, a local oscillator signal 106, a local oscillator waveguide 108, an optical coupler 110, an output waveguide 118, an optical receiver 112, and a signal processor 116. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined with a local oscillator signal. Optical heterodyne detection systems are not limited by the mechanical constraints that limit the grating based and autocorrelation based OSAs. The spectral resolution of an optical heterodyne detection system is limited by the linewidth of the local oscillator signal, which can be several orders of magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is best for the heterodyne signal to have a high signal to noise ratio. However, the desired heterodyne signal coexists with other direct detection signals. The direct detection signals include intensity noise from the input signal and from the local oscillator signal that can mask the desired heterodyne signal. One technique for improving the signal to noise ratio of the heterodyne signal involves reducing the intensity noise by utilizing two detectors to accomplish balanced detection. Although balanced detection is useful in improving the signal to noise ratio for the heterodyne signal, it has limitations.
Another technique for heterodyne signal detection described in U.S. Pat. No. 4,856,899 involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne signal. Although amplifying the input signal increases the amplitude of the heterodyne signal, the amplification also increases the intensity noise of the input signal and may not improve the signal to noise ratio of the heterodyne signal.
In view of the prior art limitations, what is needed is an optical heterodyne detection system that generates a heterodyne signal with a high signal to noise ratio.
An optical heterodyne detection system in accordance with an embodiment of the invention includes two optical receivers for separately measuring the power of an input signal and a local oscillator signal before the signals are combined. The measurements of the input signal and the local oscillator signal are then utilized to enhance the heterodyne signal to noise ratio by removing the intensity noise contributed by the input signal and the local oscillator signal. By measuring portions of the input signal power and the local oscillator signal power and then subtracting out the scaled quantities from the photocurrent measurement during signal processing, the signal to noise of the heterodyne signal is improved beyond that which is accomplished by known balanced receivers.
An embodiment of an optical heterodyne detection system includes a first receiver, an optical coupler, a heterodyne receiver, and a processor. The receiver measures a fraction of a first optical signal before the first optical signal is combined with a second optical signal and generates a first electrical signal that is representative of the measured fraction of the first optical signal where one of the first and second optical signals is a local oscillator signal and the other signal is an input signal. The optical coupler has a first input and a second input, the first input being optically connected to receive the first optical signal and the second input being optically connected to receive the second optical signal. The optical coupler has an output for outputting a combined optical signal that includes the first optical signal and the second optical signal. The a heterodyne receiver has an input for receiving the combined optical signal from the optical coupler and an output for outputting a third electrical signal that is representative of the combined optical signal. The third electrical signal includes a heterodyne signal. The processor receives the first electrical signal and the third electrical signal and generates an output signal that is indicative of an optical parameter of the input signal in response to the heterodyne signal and the first electrical signal.
In an embodiment, the processor utilizes the first electrical signal to calculate the signal noise in the third electrical signal that is contributed from the first optical signal. The processor may also subtract the calculated signal noise related to the first optical signal from the third electrical signal to improve the signal to noise ratio of the heterodyne signal.
In an embodiment, the system includes a second receiver that measures a fraction of the second optical signal before the second optical signal is combined with the first optical signal and that generates a second electrical signal that is representative of the measured fraction of the second optical signal. The processor utilizes the second electrical signal to reduce signal noise in the third electrical signal that is contributed from the second optical signal.
A method for monitoring an optical signal utilizing optical heterodyne detection involves providing a first optical signal and providing a second optical signal, with one of the first and second optical signals being a local oscillator signal and the other signal being an input signal. A fraction of the first optical signal is measured before the first optical signal is combined with the second optical signal and a first electrical signal is generated that is representative of the measured fraction of the first optical signal. The first optical signal is combined with the second optical signal to create a combined optical signal and a third electrical signal is generated that is representative of the combined optical signal. An output signal is generated that is indicative of an optical parameter of the input signal. The step of generating the output signal includes a step of utilizing the first electrical signal to reduce signal noise in the third electrical signal, wherein the signal noise is contributed from the first optical signal.
An embodiment of the method involves measuring a fraction of the second optical signal before the second optical signal is combined with the first optical signal and generating a second electrical signal that is representative of the measured fraction of the second optical signal. Additional steps involve calculating the noise that is contributed to the third electrical signal from the first optical signal utilizing the first electrical signal and subtracting the calculated noise from the third electrical signal, and calculating the noise that is contributed to the third electrical signal from the second optical signal utilizing the second electrical signal and subtracting the calculated noise from the third electrical signal.